An internal combustion engine such as a reciprocal engine of a vehicle, a ship and a generator is assembled by a cylinder block (or engine block), a cylinder head and a piston or the like. In a cylinder block, a piston is incorporated inside a cylinder, and a cylinder head is attached to the cylinder block. A combustion chamber of the internal combustion engine includes an inner wall of a cylinder provided in a cylinder block, a piston head provided on a top face of the piston, and a cylinder head. Conventionally, cast iron has been used for a material of members configuring such a combustion chamber. However, recently aluminum alloys with a less weight are more generally used therefor.
Higher output of an internal combustion engine can be achieved by increasing an engine displacement and compression rate. However, the higher compression rate becomes, the larger cooling loss of the engine becomes, which limits improvement of a thermal efficiency of the internal combustion engine. Such cooling loss of the engine generally covers approximately 30% of the thermal energy generated by the internal combustion engine. Accordingly, a decrease in the cooling loss is a key challenge for realizing an excellent internal combustion engine with higher output and lower fuel consumption.
Conventionally, a method for securing a heat-insulating property of the combustion chamber in the internal combustion engine is known available by forming a heat-shielding coating of ceramics on an inner wall of the combustion chamber, in which the ceramics are formed by firing an inorganic oxide, an inorganic carbide or an inorganic nitride. For example, ceramics such as zirconia have a high heat-resistant property and low thermal conductivity, allowing an excellent heat-resistant property of a heat-shielding coating of ceramics. However, ceramics tend to have a relatively high thermal capacity (i.e., thermal capacity per volumetric specific heat or a unit volume). This feature prevents a ceramic temperature from appropriately accommodating to change of a gas temperature in the combustion chamber.
Consequently, when such a heat-shielding coating of ceramics is used, a temperature of the inner wall of the combustion chamber is hardly lowered due to the heat filled inside the combustion chamber thus generated by a temperature rise of the inner wall during the combustion cycles of the internal combustion engine. This causes concerns of a decrease in intake efficiency and an incident of abnormal combustion.
As mentioned above, a heat-shielding coating is demanded to have performance excellent in a heat-resistant property and a low thermal conductivity as well as a low thermal capacity. Further, an inner wall of the combustion chamber is thermally expanded and contracted repeatedly during combustion cycles of the internal combustion engine, thereby to come under a strong combustion pressure generated by combustion gas. Therefore, such a heat-shielding coating is further demanded to have enough adhesivity hard to be peeled from the inner wall of the combustion chamber.
Hence, the following technologies have been developed for realizing a heat-resistant coating having a low thermal capacity and excellent adhesivity, for example, by forming an anodic oxide coating or a porous spray film on the inner wall of the combustion chamber. Further, other technologies are also proposed to form a heat-shielding coating by including hollow ceramic particles with a low thermal capacity in a metallic portion.
For example, Patent Document 1 discloses a heat-insulating structure including a hollow particle layer formed by densely filled with a number of hollow particles on a surface of the metallic base material, and a coating layer formed on the hollow particle layer. Further, it is disclosed that the hollow particle layer is formed by brazing the base material with a hollow particle mold prepared by bonding the hollow particles each other via pulse discharge firing or hot-molding with a binder film (see paragraphs 0046, 0051), and the coating layer is formed of metal or a metal oxide (see paragraphs 0041-0042).
Further, Patent Document 2 discloses a heat-shielding coating formed on a surface of a metallic base material. Herein, the heat-shielding coating is formed by bonding a plurality of ceramic hollow particles with a metallic portion via point jointing. Moreover, it is disclosed that the heat-shielding coating is formed by preparing slurry via mixing ceramic hollow particles with metal particle paste made of at least metal particles and a solvent, applying the slurry onto a surface of the metallic base material, heating the applied slurry at least at the temperature equal to or more than the boiling point of the solvent to evaporate the solvent, and further heating the slurry at least at the temperature equal to or more than the melting point of the metal particles to melt the metal particles so that the resulting molten metal coalesces between the plurality of hallow particles (see paragraphs 0049-0054).